Hydrogen often contains impurities that must be removed before it can be used in many applications, such as for producing power in PEM fuel cells. Today and in the immediate future, most hydrogen is produced via steam reforming of natural gas, so most common impurities are water vapour (H2O), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4) and Nitrogen (N2). Of these impurities, CO is the most potent and it can rapidly poison metal hydride alloys, as well as fuel cell electrode catalysts.
Impurities interact with hydride alloys with varying effects such as follows:                Poisoning results in a rapid decrease in hydrogen capacity with cycling. Damage from poisoning tends to be localized on the alloy particle surface, so it is often possible to restore performance with little, if any, loss in capacity. Oxygen and CO are impurities that poison alloys.        Retardation is manifested by a reduction in absorption kinetics without loss in ultimate capacity. With enough time, full capacity can be achieved. Retardation is often a result of impure species forming weak bonds on the alloy surface that impede hydrogen absorption. Carbon monoxide and water vapor retard hydriding.        Reaction causes irreversible capacity loss through bulk corrosion of the alloy. Reaction results in the formation of very stable chemical compositions that do not reversibly hydride and cannot be easily returned to their original state. Active sulfur compounds such as S02 cause irreversible damage due to the reaction mechanism.        Innocuous Blanketing results from impure gas species congregating on the alloy surface and forming a blanket that reduces the rate of hydrogen absorption. Gases that cause blanketing, such as nitrogen and methane, are easily removed by venting.        
Water vapor is the most common contaminant in hydrogen. It affects hydride alloys through the mechanisms of retardation followed by poisoning. When hydrogen absorption begins, water vapor is carried towards the alloy surface by the hydrogen. The water molecules gather and become concentrated on the alloy surface, slowing the passage of hydrogen to the alloy (retardation). Hydride alloys contain nickel, which normally acts as a dissociation catalyst for hydrogen molecules prior to absorption. The nickel can also act as a weak catalyst for dissociating water molecules. As water molecules are dissociated at the surface of the alloy particles, the resulting hydrogen would be absorbed into the alloy, but oxygen tends to react with the rare earth element (lanthanum or mischmetal) forming a stable oxide that is no longer available to hold hydrogen. Hydrogen absorption capacity decreases (poisoning). Nickel-metal hydride battery alloy development by the applicant and others stimulated a large body of research into the corrosion of hydride electrodes immersed in electrolytes.
Previous teachings of disproportionation resistant alloys and of methods that inhibit disproportionation of hydrides are described in commonly owned U.S. Pat. No. 6,508,866 “Passive Purification in Metal Hydride Storage Apparatus” issued on Jan. 21, 2003, and also of U.S. Pat. No. 5,673,556, entitled “Disproportionation resistant metal hydride alloys for use at high temperatures in catalytic invention incorporates by reference the previous teachings described in Applicant's U.S. Pat. No. 6,508,866 “Passive Purification in Metal Hydride Storage Apparatus,” issued on Jan. 21, 2003, and also U.S. Pat. No. 5,673,556, entitled “Disproportionation Resistant Metal Hydride Alloys for Use at High Temperatures in Catalytic Converters,” issued on Oct. 7, 1997. Both of these patents are commonly owned by Ergenics Corporation of Ringwood, N.J., USA, the assignee of the present invention.
A number of corrosion inhibiting additives, such as cobalt and tin, have been identified for immersed alloys, and we have found these to have positive impact on preventing poisoning in gaseous systems as well. U.S. Pat. No. 6,508,866 describes a method of removing vapor and oxygen from hydrogen within a hydride alloy bed. This in-situ purification process permits the alloy to operate as if it was absorbing clean, dry hydrogen. Named “Passive Purification”, the process includes catalytic recombination of oxygen impurities, physical water removal and the use of corrosion inhibitors in the alloy formulation. During desorption of dry hydrogen from the alloy, water that was removed during the absorption process is evaporated into the hydrogen as it exits the bed. The Passive Purification process successfully permits cycling hydride alloys with hydrogen that is saturated with water vapor and contains some oxygen.
It has been suggested, in Sandrock, G. D. 1997. “State of the Art Review of Hydrogen Storage in Reversible Metal Hydrides for Military Fuel Cell Applications,” Office of Naval Research. Ringwood N.J.: SunaTech, Inc., that a single monolayer of Carbon Monoxide (CO) and, to a lesser degree, Carbon Dioxide (CO2) prevents hydrogen absorption into hydride alloys, probably by forming Ni-carbonyl bonds on the alloy surface which deactivate the dissociative properties of the nickel. Sandrock shows that there is almost no degradation of performance in cycling at a temperature of 115° C. LaNi5 alloy in hydrogen contaminated with CO. This suggests that an “elevated temperature desorption” can remove the CO molecule from the alloy to restore full performance.
Nitrogen, methane, noble gases and ammonia do not react with hydride alloys, but if present in enough quantity, can form an innocuous blanket, which reduces the rate of the hydrogen absorption to a crawl. Intuitively, during a desorption cycle, hydrogen that is released from the hydride alloy particles should be able to sweep away impurities that cause innocuous blanketing and they can be either removed from the hydrogen stream by an automatic venting process or be allowed to pass through the compressor.
One application of hydrogen is its use in hydride compressor systems. In such systems, hydrogen is generally absorbed in a reversible metal hydride alloy at low pressure in a hydride bed which is subsequently heated and hydrogen is released at high pressure. Continuous compression is achieved by having two containers/hydride beds in a parallel configuration. One being cooled by water while hydrogen is absorbed until it is full while the other is heated with hot water to release the hydrogen. With each thermal cycle, the alloy in the containers are first filled to capacity and then emptied. Gaseous impurities, within the hydrogen stream, can react with the hydride alloy and reduce its hydrogen storage capacity and/or impede the absorption of hydrogen. The result will be a decline in hydrogen throughput with each thermal cycle. Generally, for this reason, thermal compression of hydrogen using metal hydrides has been restricted to relatively pure hydrogen streams (99.995%) that have less than 50 ppm of active gas impurities.
Presently hydrogen is purified with Pd alloys or oxygen and water scavengers as well as with filters which selectively retain contaminants and let hydrogen pass through. All these methods involve high cost and can only be implemented in stationary distribution centres. In fact the quality of hydrogen to be sold in the different stationary distribution centres can vary appreciably from one to another because of the different production methods and purification methods used. This can only impair the confidence of end users in the future as the reliability of the hydrogen source can be unpredictable. In addition, fluctuations in the quality of hydrogen may be anticipated up to the point that the performance of vehicles could be reduced.
In commonly owned U.S. Pat. No. 5,673,556, entitled “Disproportionation resistant metal hydride alloys for use at high temperatures in catalytic converters,” hydrogen purification systems may be used to remove impurities. However, such purification systems are often complex, expensive to maintain, and, for hydrogen produced at atmospheric pressure, would require their own motive force in the form of a mechanical compressor or blower. The present invention offers a simple, compact and mobile purification system and method for hydrogen purification.